I can understand your situation, too. I was living with parents and siblings who smoked...very frustrating on top of the added health risks.
I had one time where we all were sitting in the living room watching tv, and my dad would light-up, and when he smoked his, my one brother would then light-up, and when he was done, number two brother would light-up! When I saw my dad light up another one, I exclaimed "Why don't you all just smoke at the same time!" then I left the room...
Many years later, my dad had apologized repeatedly about all the times he had exposed me to his cigarette smoke...maybe something can be done where your parents can apologize much sooner?
Only one effective method I can think of is to demand they don't smoke around you...I suppose you can't help it with them smoking. Though if you showed them enough scary pictures of what smoking does to the lungs, maybe that would change their tune..and show them how much money they'd save if they quit smoking.
Sorry if that doesn't help, but hope you can find a solution.